libbacktrace: make Backtrace::FormatFrameData static.

Backtrace::FormatFrameData doesn't depend on anything from its
instance. Make it static so that callers that don't have a Backtrace*
available can format frames.

Test: mma
Change-Id: Id0ca93bbef19da488305374e58b25c7ca9d0e518
diff --git a/libbacktrace/include/backtrace/Backtrace.h b/libbacktrace/include/backtrace/Backtrace.h
index d67ea50..289fd0c 100644
--- a/libbacktrace/include/backtrace/Backtrace.h
+++ b/libbacktrace/include/backtrace/Backtrace.h
@@ -127,7 +127,7 @@
   // Create a string representing the formatted line of backtrace information
   // for a single frame.
   virtual std::string FormatFrameData(size_t frame_num);
-  virtual std::string FormatFrameData(const backtrace_frame_data_t* frame);
+  static std::string FormatFrameData(const backtrace_frame_data_t* frame);
 
   pid_t Pid() const { return pid_; }
   pid_t Tid() const { return tid_; }